Soft Microdenticles on Artificial Octopus Sucker Enable Extraordinary Adaptability and Wet Adhesion on Diverse Nonflat Surfaces

Abstract Bioinspired soft devices, which possess high adaptability to targeted objects, provide promising solutions for a variety of industrial and medical applications. However, achieving stable and switchable attachment to objects with curved, rough, and irregular surfaces remains difficult, particularly in dry and underwater environments. Here, a highly adaptive soft microstructured switchable adhesion device is presented, which is inspired by the geometric and material characteristics of the tiny denticles on the surface of an octopus sucker. The contact interface of the artificial octopus sucker (AOS) is imprinted with soft, microscale denticles that interact adaptably with highly rough or curved surfaces. Robust and controllable attachment of the AOS with soft microdenticles (AOS‐sm) to dry and wet surfaces with diverse morphologies is achieved, allowing conformal attachment on curved and soft objects with high roughness. In addition, AOS‐sms assembled with an octopus‐arm‐inspired soft actuator demonstrate reliable grasping and the transport of complex polyhedrons, rough objects, and soft, delicate, slippery biological samples.


Detailed Derivation of AOS-sm Attachment on Substrates Under Dry Conditions
The artificial octopus sucker with soft microdenticles (AOS)-sm first induces contact with a substrate and then undergoes structural changes through pneumatic actuation that produce a suction-based attachment force. The dry attachment force (F s,dry ), which is the suction-based attachment force of AOS-sm under dry conditions, [1] can be expressed as follows: where is atmospheric pressure (~101.3 kPa), is an experimentally determined constant between 0 and 1 that compensates for seal leakage at the AOS-sm regions of contact, is the diameter of the interfacial area when a vacuum is established in the inner chamber, is the diameter of the upper portion of the inner chamber in the vacuum state, is the chamber height under vacuum, is the diameter of the upper portion of the inner chamber during initial protuberance contact, is the diameter of the interfacial area of the AOS chamber during this phase, and is the height of the chamber during this phase. To account the adhesive interactions of the microdenticles, is induced by the van der Waals interactions between the soft microstructured AOS rim and the substrate under dry conditions, [1] and A' is the effective interfacial area between the AOS rimsubstrate interface, as shown in Figure S4d. The experimental values of , , , and were measured using a Vernier scale, whereas and were measured using a compact laser displacement measurement sensor. The theoretical values of , , , , , and were obtained from a finite element method (FEM) simulation.

Derivation of AOS-sm Attachment on Substrates Under Wet Conditions
In a wet environment, the suction-based attachment force is obtained by calculating the simple volume change upon deformation and the actual contact area. It can be assumed that there is little internal pneumatic pressure when the internal volume of AOS-sm changes in a wet environment. As the inner pressure is zero, the pressure difference between the interior and exterior of AOS-sm can be maximized, expressed as . Hence, the suctionbased attachment force of the AOS-sm under wet conditions ( ) is as follows: .
(S2) Similarly, the experimental and theoretical values of were substituted into Equation (S2) to understand wet adhesive performance.

Derivation of Compensation Factor on Rough Surfaces with R a
The suction-based attachment force of AOS-sm is determined by , , and a compensation factor ( ) according to pressure leakage, as shown in Equation (2). Among these factors, and depend on the structural changes of AOS-sm under pneumatic operation, whereas is related to interfacial interactions between the infundibulum of AOSsm and the object; [2] γ increases as the interaction between the surface and the infundibulum of AOS-sm is weakened and decreases as the interaction is strengthened. This correlation can be interpreted based on the adhesive force data for each structure of the infundibulum according to the surface roughness. As shown in Figure S7, the dependence of the adhesive force on the roughness can be expressed using the following linear approximation: where r is ratio of the reciprocal adhesion forces (r = / , is the interaction force of infundibulum with a flat substrate, is the interaction force of the infundibulum with a rough substrate with R a , and is the leakage parameter determined by the induction between each infundibulum and the substrate surface. Assuming that the ratio of total suction-based attachment force of AOS-sm is proportional to the interaction force of the infundibulum, , where is the suction-based attachment force of AOS-sm on a flat substrate, and is the suction-based attachment force on a surface with a roughness of R a . As these forces can be expressed as , , this expression can be rewritten as follows: where is the compensation factor on a rough substrate with R a , α is the adaptability constant, R a is the average roughness of the substrate, and is the compensation factor on a flat substrate. This relationship can be expressed with respect to ( ) as follows: (S6) Therefore, the suction-based attachment force of AOS-sm considering the suction-based attachment force due to structural changes ( ) and interfacial interactions ( ) between the infundibulum and the surface can be expressed as follows.

FEM Simulation
FEM simulation was performed using commercial software (COMSOL Multiphysics version 5.2a, ALTSOFT, Republic of Korea, license number: 5084832). Custom 2D models with sizes of 500 × 510 mm 2 were built. The model is automatically tangent to the tetrahedral surface. The stress and strain on the structure were analyzed using the structural mechanical module using the following equations: (S10) where u is the displacement of each point,  is the infinitesimal strain tensor, C is the elasticity tensor, is the stress tensor, and F is the external force. For simplicity, pseudostatic analysis and a linear elastic material model were used. The elastic modulus was set at 300 kPa and Poisson's ratio was set at 0.49 for this specific model.

Fabrication of Microdenticle Mold
As shown in Figure  Minuta Tech, Republic of Korea) droplets were dispensed onto the master, and a polyethylene terephthalate (PET) film (50 m) was pressed lightly against the liquid droplets as a supporting backplane. Following this partial wetting of the prepolymer, air bubbles were trapped in the microhole chambers owing to the viscosity and interfacial tension of s-PUA. [3] After UV exposure, the s-PUA master was affixed to a glass substrate, and a polydimethylsiloxane (PDMS) mixture (Sylgard 184, Dow Corning, USA) with 10 wt% curing agent was spin-coated on top of s-PUA at 300 rpm for 1 min. After thermal curing at 70 °C for 2 h, the PDMS master was placed in a vacuum chamber (~10 −1 Pa) with 1 mL of FOTCS. The chamber was heated at 50 °C for 2 h for vapor deposition of the SAM reagents onto the samples, followed by annealing at 80 °C for 30 min.

Characterization of Microdenticles and Substrate Roughness
To observe the morphologies of the microdenticles and rough substrates, scanning electron microscopy (SEM) images were collected using a field-emission scanning electron microscope (7600F, JOEL, Japan) at acceleration voltages of 5-15 kV and a working distance of 8.0 mm. The samples were coated with platinum before analysis to prevent electron charging. Optical microscopy (OM) images of the patch samples were collected using a light microscope (BX51, Japan). For each point, the vertical distance ( ) from the mid-line of the surface step height corresponding to the horizontal displacement was measured and the average roughness was calculated using ∑ .

Fabrication of Rough Substrates
To obtain s-PUA substrates with rough morphologies (R a = 40, 80, 162, and 200 m), a PDMS mixture with 10 wt% curing agent was poured onto sandpaper of varying roughness (FEPA Grit Designations P360, P180, P100, and P80, which correspond to average grain sizes of 40.5, 82, 162, and 201m, respectively) (see Figure 4d). After thermal curing at 70 °C for 2 h, the cured PDMS roughness mold was peeled off the sandpaper. Droplets of the s-PUA prepolymer were then dispersed evenly onto the PDMS mold and a PET film was pressed lightly against the liquid droplets as a supporting backplane. The s-PUA was exposed to UV light for 2 min, and the PET film with a rough s-PUA substrate was peeled off. The s-PUA replica was further exposed to UV light for 12 h to achieve complete curing ( Figure   S19).

Fabrication of Curved Substrates
Surfaces with various curvatures and a height of ~3 cm were designed using 3DCAD software.
Upon fabrication, flat s-PUA substrates (area ~ 4 × 4 cm 2 ) were laminated onto the curved surfaces so that all measurements were performed using the same substrate materials ( Figure   S19). Herein, the curvature of the substrates is defined by the AOS-sm-to-surface curvature ratio (r/R), where r/R < 1.

Fabrication of Octo-Gripper
The 3D-printed molds for the octo-gripper arm were designed via CAD drawings. The molds were treated in a SAM solution of 1% octadecyltrichlorosilane(ODTS) in hexane for 1 h at ambient temperature and then heated in an oven for 12 h. Subsequently, the inner and outer molds for the inflatable top portion were assembled, and the Dragon Skin 10 prepolymer was deposited within 3D-printed molds ( Figure S16). After curing at room temperature for 4 h, the top portion was demolded from the assembly. A piece of paper, which served as the strainrestraining layer, was placed into the mold for the bottom portion before adding Dragon Skin 10. The top portion of the octo-gripper arm was positioned inside the bottom layer mold before curing, so that the prepolymer was cured in place. After the assembly was cured at room temperature for 4 h, three AOS-sms were arrayed onto the surface of the bottom portion of the octo-gripper arm and fixed using a silicone adhesive ( Figure S16). Finally, curing at room temperature for 6 h produced the integrated octo-gripper.

Actuator (OASA)
As shown in Figure

Practical Demonstrations of Object Transfer Using AOS-sms and the OASA
A commercial open-hardware oriented platform (OpenMANIPULATOR-X RM-X52-TNM, ROBOTIS, Republic of Korea) was utilized to demonstrate the practical application of the AOS-sms and OASA to the highly adaptable attachment and transport of various objects. The AOS-sms and OASA were connected to the manipulator using a 3D-printed design, and a conventional Arduino board was used to transmit signals from a computer.

Transfer Demonstrations with Porcine Liver and Heart
Fresh porcine liver and heart (postmortem) organs were obtained from a local slaughterhouse                     Table S1. Adhesive performances of previously-developed bioinspired adhesive actuators in dry/wet environments and comparison to our work.